Refrigeration systems or evaporative systems for cooling are well-known in the art. Refrigeration is the process of removing heat from a substance or space in order to lower its temperature. To extract heat energy from the air, the air is placed in contact with a material at a lower temperature so that heat flow will occur in a descending temperature gradient. The low-temperature material is usually either a cold metal surface or a chilled-water spray. In either case, the working substance of the system is an evaporating refrigerant in a direct-expansion cooling coil or in the tubes of a water chiller. The energy absorbed is rejected typically to the outdoors through an air-cooled condenser or cooling tower. The present invention is presented in the context of cooling the airflow to an air-cooled condenser, which is a common means of heat rejection.
The refrigerants, which are certain low-boiling-point substances, are used as the working fluid or heat-transfer media of typical refrigeration systems. They are used in a cyclical thermodynamic process that involves two changes of state: between liquid and vapor and back. An example of a compression refrigeration cycle that uses a direct-expansion cooling coil is now presented.
Referring to FIG. 1, there is shown a basic compression refrigeration system 10 that has a closed refrigerant loop that is used in a compression refrigeration cycle. In this cycle, there is an alternate compression, liquefaction, expansion, and evaporation of the refrigerant. The air to be cooled is shown symbolically by arrow 12 at an initial temperature of TA1 and flows across an evaporator 14 that removes heat from the air to produce a cooled air represented by arrow 16. The cooled air 16 is at a temperature TA2, where TA2<TA1.
The evaporator 14 serves as the heat sink for removing the heat from the air 12. The refrigerant vaporizes there as it absorbs the heat that is removed from air 12. The evaporator 14 may take one of several forms. The evaporator 14 may be an extended surface (or finned) cooling coil with a direct-expansion system or the heat exchanger coils of a water chiller for chilled water systems.
The heat in air 12 is delivered to the refrigerant in evaporator 14 and the refrigerant, which is then at a pressure of PR2 and a temperature of TR2, is delivered to compressor 18. The compressor 18 is a device for accomplishing primarily two functions. First, it removes vapor from the evaporator 14 at a rate that permits steady state conditions of low temperature and low pressure in the evaporator 14. Second, the compressor 18 discharges the vapor at a pressure (PR3) and temperature (TR3) high enough to permit heat rejection along a descending temperature gradient to the air or water of the condenser 20.
In the condenser 20, the heat originally removed from air 12 plus the heat equivalent of the work performed in the compressor 18 are rejected to the condenser coolant (air or water) and ultimately to the outside air or earth. The compression and removal of the heat from the refrigerant operate to return it to a liquid state at the condenser pressure, and the liquid refrigerant is collected by liquid receiver 22. From there, the refrigerant is delivered to an expansion valve 24. The expansion valve 24 produces a sudden drop in refrigerant pressure (i.e., PR4>>PR1) and that in turn creates a sudden drop in temperature, TR4>>TR1. And it regulates the flow of refrigerant producing a uniform evaporating temperature for evaporator 14.
In a typical packaged air conditioning (rooftop unit) or split air conditioning system, the compressor 18 and condenser 20 are located in a single unit outside the house or building. The compressor-condenser unit has a hermetically sealed compressor and motor in the middle of finned-tube air-cooled condenser forming the sides of a u-shaped (or similar) housing. The unit has a condenser fan and motor located on a top portion of the housing to provide a flow of outside, ambient air across the condenser fins and out of an open top portion. The size of systems varies according to the cooling needs.
The cooling load on a space to be conditioned is substantially linear on a graph if the cooling load is placed on the ordinate and the outside temperature on the abscissa. Typically an air conditioning unit's cooling capacity versus temperature is also nearly linear on the same graph with high cooling capacity at lower outdoor temperatures and less cooling capacity at higher temperatures. For example, a Carrier 48TJ006 (5 Ton) unit developing air at the evaporator at 67 F will have a cooling capacity of 65.5 MBtu/hr. at 85 F, but only 56.5 at 105 F, which is a drop of about 14% capacity as the outside temperature went from 85 to 105 F. There is also about a 13% increase in power consumption at the higher outdoor temperature. Cumulatively, there is a reduction in efficiency of about 24%. This type of information is used to size air conditioning systems for a given space and conditions.
The American Society of Heating, Refrigeration and Air-Conditioning (ASHRAE) provides guidelines for helping to size a unit for a given application. ASHRAE set the standards by which one sizes unless an ordinance requires otherwise. To size the air conditioning unit, the intersection of the linear (or nearly linear) capacity of the air conditioner with the linear loading profile of the space is located for the maximum design temperature for the outside, ambient air. If a system goes above the maximum temperature for which the unit was sized, the air conditioning unit will never catch up and cool the space down to the desired temperature. To conservatively size the unit to handle the hottest days of the year, a substantial amount of the required capacity is needed just for the hottest days. For example, a 5-ton unit might be needed to handle the hottest days, but in fact a 4-ton unit would do well for the vast majority of the year. It would thus be nice to size a unit to handle the majority of the temperature range without being unduly influenced by the end point—i.e., the hottest days.
Numerous efforts have previously been made to enhance the design of air conditioning systems and particularly air conditioning systems that reject heat through air cooled condensers. Some approaches have involved adding supplemental refrigerant coolers and some that have tinkered with the condenser cooler itself. For example, U.S. Pat. No. 5,553,463 describes a system that includes a supplemental condenser for cooling the refrigerant with a closed-water loop that has a cooling tower. The refrigerant is cooled some by the closed-water system before going to the condenser coil. Improvements remain desirable.